1. Field of the Invention
This invention relates to multi (i.e. two or more) part sets of sheet material, which when a set is used provide for so-called "carbonless" copying or imaging between parts of the set, whereby impact printing of an image on an upper sheet of the set will simultaneously make similar images on a or the lower sheet or sheets of the set, and wherein at least some of the sheets of a set may bear a pre-applied image.
The sheet material usually is paper and herein will generally be referred to as such; however it is to be understood that the invention is applicable to the use of other suitable sheet material.
2. Description of Prior Art
Multi-part sets of so-called "carbonless" papers are well known. They rely on two coatings formed respectively on the contiguous faces of superimposed sheets. The coatings usually are one containing a colour-forming substance, usually contained in micro-capsules, on the back (underside) of an upper sheet (usually known as a CB coating) and a coating of a receptor layer on the front (upper side) of a lower sheet (usually known as a CF coating). Colour-forming chemicals are typically dissolved in an oily solvent and encapsulated by well known techniques, and when micro-capsules thus produced are ruptured by mechanical pressure, as by impact in a printing process involving mechanical impact, the chemicals are released and react to form a visible mark on the CF coating of the adjacent sheet. One or more sheets having a CB coating on the underside and a CF coating on the upper side (known as a CFB sheet) may be interposed between uppermost and lowermost sheets having CB and CF coatings respectively on their underside and upper side as aforesaid. An image may be pre-applied to the upper side of each sheet by a suitable printing process (e.g. offset, ink jet or toner deposition) which does not damage the coating on the sheets.
Most commonly, such multi-part sets have been made by preparing the sheets individually and then securing them together in sets by the use of adhesive. The collating of individual sheets to form sets and use of adhesive to secure them together in the sets is time consuming and expensive. It can require special coatings to ensure the adhesive will only adhere the sets together. The operation requires skill and experience, and results are not always reliable. Any problem arising at this stage might entail the wastage of material which has already been subjected to relatively costly treatment, if the problem cannot be rectified.